1. Field of the Invention
The present invention relates generally to a nitrogen oxide reducing system for a diesel engine and a nitrogen gas generating device for reducing a nitrogen oxide in an exhaust gas by adjusting a nitrogen concentration in induction air. More particularly, the present invention relates to a system for preventing oxidation of nitrogen by mounting a nitrogen gas generating device on a mixer for mixing air in the mixer with nitrogen supplied from the nitrogen gas generating device for increasing nitrogen concentration.
2. Description of the Related Art
Currently, as a part of environment protection policy, restriction for discharge of exhaust gas from a diesel engine for suppressing air pollution is becoming more and more strict year by year. Particularly, even in a large size diesel engine to be employed in a special purpose vehicle, such as a construction machine, industrial machine or the like, severe regulation is being about applied.
Conventionally, as one of causes of air pollution, various proposals have heretofore been made for reducing nitrogen oxide contained in the exhaust gas of the diesel engine.
As one of the proposals, so-called exhaust gas recirculation (EGR) to recirculate a part of exhaust gas to induction air to lower combustion temperature to restrict generation of nitrogen oxide, has been widely known.
On the other hand, as disclosed in Japanese Patent Application Laid-open No. 2(1990)-191859, there has been disclosed an air induction system of the diesel engine including a separator having an oxygen selective transmission wall having an oxygen separating function, a device for sucking oxygen rich air via the oxygen selective transmission wall of the separator, and means for sucking air having low oxygen concentration upstream of the oxygen selective transmission wall in a partial load condition to the engine, adjusting oxygen concentration of suction air depending upon driving conditions for restricting combustion in the partial load condition of the engine for reducing generation of nitrogen oxide.
Furthermore, a system for removing nitrogen oxide by means of a catalyst has also be proposed.
However, among the prior arts, the exhaust gas recirculation system introduces the exhaust gas per se into the induction air so that particulates, sulfur oxide and so on contained in the exhaust gas are inherently introduced into the engine to degrade a lubricant oil and to accelerate wearing of sliding portions in the engine. Therefore, in view of durability and combustion ability of the engine, it should be a problem to perform exhaust gas recirculation in a sufficient amount for restricting generation of nitrogen oxide. Therefore, in the diesel engine having the exhaust gas recirculation system, non of the system has been successfully adapted to the current regulation.
On the other hand, in the air induction system for the diesel engine as disclosed in Japanese Patent Application Laid-Open No. 2 (1990)-191859, problems in the exhaust gas recirculation will not be encountered since the oxygen rich air is sucked through the oxygen selective transmission wall of the separator by a suction pump and air having low oxygen concentration as removed oxygen is sucked into the engine, and the exhaust gas per se is not introduced into the induction air.
However, oxygen molecule transmission hole of the oxygen selective transmission wall formed from oxygen enrichment membrane using high molecular material, is in an order of micron. Therefore, even though oxygen is sucked by means of the suction pump utilizing negative pressure, since transmission resistance of the oxygen molecule is large and sufficient pressure difference cannot be obtained by the suction pump utilizing negative pressure, a sufficient amount of induction air, oxygen concentration of its low enough to restrict generation of nitrogen oxide, is practically impossible to instantly obtain in a distance of short separator forming a part of the suction passage. As a result, it has been practically impossible to introduce a sufficient amount of induction air with low oxygen concentration sufficient for restricting generation of nitrogen oxide without causing lowering of engine output, and thus is impractical. Therefore, it is clear that restriction cannot be satisfied in this system.
Furthermore, the system employing a catalyst cannot achieve sufficient effects. Therefore, currently, no system can satisfy the current restriction value.